For me there’s two separate participants, a ‘talker’ and a ‘listener’. My mind identifies more with the talker, because that’s the one that has agency. Since there are two participants, both of which are me, I talk in 1st person plural (‘we’ve got to do …’, 'we thought about this earlier’). I stopped being afraid of being alone after I started having an internal dialogue around the age of 11, since having a second participant in the conversation meant I was always in company.
Edit: Wow, looks like there’s a lot more diversity in this than I was expecting
Mine is constantly going off all the time, like I’m preparing an argument of some sort (which usually has nothing to do with what I need to be doing) and need to process all the information I can over and over again to make sure everything lines up. If someone talks for more than 5 seconds my own internal monologue kicks in until the person stops or like 30 seconds pass and I realize it distracted me. Makes it difficult to read as well, and on bad occasions it can last for hours. The only time this doesn’t happen is if I’m intensely focused on something like a self-driven programming project.